This first-time discovery of carbonates in an underground
location points to a warmer epoch in the ancient Martian past with more
atmospheric carbon dioxide, as well as ancient
seas. The carbonates also turned up alongside silicate minerals and clays that
suggest the presence of hydrothermal systems similar to the deep sea vents on
Earth.

"This discovery doesn't really hint at life, but it
does reveal a very strong candidate for a habitable environment, perhaps
the best discovered so far," said Paul Niles, a planetary geologist with
the NASA Johnson Space Center in Houston.

The ancient rocks may date back as far as 3.5 or 4 billion
years, according to Joseph Michalski, a planetary geologist at the Planetary
Science Institute in Arizona. That means they don't necessarily reflect the
Martian underground as it exists today, but they do point to processes which
could make the Mars subsurface a habitable place.

Researchers have long looked to the underground environment
in hopes of finding Martian life, because cold, dry conditions coupled with ultraviolet
radiation on the surface of Mars makes existence up above unlikely.

"The subsurface environment provides a warm,
stable environment that should be conducive for life to evolve given our
limited understanding of that process," Niles said in an e-mail.

The research is detailed in the Oct. 10 issue of the journal
Nature Geosciences.

The methane mystery

The findings may also prove a huge step toward solving the mystery
of methane gas on Mars today. Such gas could originate from either
methane-burping microbes or non-organic geological processes, but so far no
signs of life have turned up as the culprit.

A recent study showed that methane
disappears from Mars within less than a year another sign that something
must be constantly creating the methane on the red planet.

But now researchers may have a geological, rather than
biological, answer.

Hydrothermal processes with warm liquid water can chemically
interact with rocks in a way that produces methane, and the recent Leighton
Crater study has laid out strong evidence of hydrothermal systems having at
least existed on Mars.

A triumph of the geological explanation does not
automatically rule out the chances for life, said Michalski, who coauthored the
recent study with Niles. That's because hydrothermal systems would make the
underground Martian environment an even more attractive-looking place for life
to hide out.

"That is what's cool about Martian methane; it either
formed organically or in an environment that's conducive to habitability,"
Michalski told SPACE.com.

Scratching the surface

Finding out more about the Martian underground has been difficult
because of a volcanic upper crust that hides what lies beneath. The Leighton
Crater itself sits just southwest of a giant dormant Martian volcano named
Syrtis Major, between the northern lowlands and the southern highlands.

If researchers can find other craters that provide more
windows into the underground scene, they could confirm a sneaking suspicion
about carbonates being spread across a large region of Mars. Such minerals
could prove invaluable for their record of water's history that dates back to
the warmer, carbon dioxide-rich period of ancient Mars.

"The difficulty with Mars is we only see the surface
layer for a long time [because of a lack of plate tectonics]," Michalski explained.
"The best way to look at the deeper part of the crust is to look at impact
craters several kilometers down."

Future robotic missions beyond the Mars Science Laboratory,
NASA's SUV-sized rover slated to launch in 2011, could also target areas
similar to the Leighton Crater so that they can study the chemical makeup of
the rocks in detail.

Meanwhile, the Mars Reconnaissance Orbiter's instruments
the Compact Reconnaissance Imaging Spectrometer for Mars and High Resolution
Imaging Science Experiment continue to pay off for researchers more than five
years since the probe launched.

"These missions are part of a big group effort,"
Michalski said. "Even though there are just two authors on the paper, we
never want to give impression that we're working alone."